Different types of imaging techniques such as positron emission tomography (PET), magnetic resonance imaging (MRI) and ultrasound imaging are available that can non-invasively gather information from within biological tissues as a basis for image reconstruction. More recently, another imaging technique, namely optical imaging has been the subject of intense research and commercial development.
Optical imaging is based on the information that can be derived from the analysis of the signal resulting from the interaction of light with matter as it is propagated within an object. Optical imaging of turbid medium can be performed using three different approaches namely continuous wave (CW), time domain (TD) and frequency domain (FD). CW is the simplest and least expensive of the three techniques but provides only limited information with regards to the spatial distribution of internal optical attenuation of the object being imaged. TD and FD, by conveying information on the time required by photons to travel within the object (FD through the Fourier transform) are considered to be “time resolved” and can be used to calculat the spatial distribution of optical characteristics of the object, such as absorption and scatter coefficients, via well known photon diffusion equations (for a review paper on this topic, see the article by Hawrysz and Sevick-Muraca, Neoplasia, vol.2 No 5, pp388-417, 2000).
Optical imaging is particularly attractive in view of its non-invasiveness which permits the acquisition of in vivo information without damaging biological tissues. Furthermore the technique may be useful to monitor drug distribution, detect the presence of abnormalities within organs, or map physiological activities within mammals.
However, widespread utilization of optical imaging systems has been impeded by some undesirable characteristics of existing systems. For example, optical imaging devices often require cumbersome arrangements of optical fibers that are used to transport the light to and from the object. Such systems have been described for example by Ntziachristos and Weissleder in patent application WO 02/41769 and by Hillman et al. Phys. Med. Biol, 46 (2001)1117-1130. The type of arrangement for the optical components described in these references requires a time consuming alignment of the region of interest with the optic fibers used to illuminate the object and detect the optical signal. This type of arrangement is particularly problematic when imaging is performed on living tissues of mammals.
Ease of data acquisition and in particular ease of the positioning the object relative to the optic components is especially important in applications requiring high throughput such as in clinical settings or in research that make use of small mammals such as mice. In this respect, commercially available optical imaging systems for imaging small mammals have been developed. For example, a bioluminescence imaging system developed by Xenogen Corp. (Biophotonics, vol.9, No.7 pp48-51, 2002) has been designed to collect light emanating from small mammals. However, this imaging device suffers from certain disadvantages. For example, it requires the presence of bioluminescent molecules which have a spatially restricted biodistribution profile therefore greatly reducing the flexibility in imaging desired region of interests (ROI). Furthermore, the technique is limited by the number of luminescent molecules that are currently available. Moreover, the system does not allow time resolved data to be acquired.
In view of the above, it would be desirable to provide an optical imaging system for imaging turbid media such as biological tissues that allows time resolved optical data to be acquired with increased flexibility and efficiency.